Since QCD is an ever growing area of physics, a number of sections in this third edition of the book have been expanded, in particular, those dealing with deep inelastic scattering. Here new material is included both on higher-order calculations, quite a number of which have become available in the last few years, and on the small-x limit of structure functions, where, triggered by the results of HERA, there has been considerable activity. Two other topics have also received special attention. One is the matter of bound states of heavy quarks, where inclusion of higher-order perturbative and nonperturbative corrections has led to a clarification of the QCD description, particularly for the lowest states of heavy quarkonia. The other concerns the results of high-order (four-loop) calculations and a new determination of the parameters of the theory, quark masses, etc. The emphasis is on detailed calculations, thus making the book particularly useful for students and experimentalists.